Hey Felix, where did you find BOM for these modules?
Anyway I got replacement modules today in the mail (this time it's RFM69W) and soldered to same PCB where I had issues before, and everything works great at 915Mhz! Even without any antenna attached
The SX1231H datasheet is a good place to start. Then I reverse engineered the Hope version, they add a bunch of passives for their output routing path. This is because they use the same PCB for many different versions of the RFM69/65.
I would recomment soldering the antenna otherwise you might risk reflecting that signal back into the transceiver and potentially damage it if you run it like that for a longer time.